Commercial resin composites work well in anterior restorations. However, in posterior restorations the clinical failure rate is 25% or more after 10 years with fracture being one of the two key contributors to failure. This program seeks to improve the clinical performance of resin based posterior restorations by developing a new class of dental composites that mimic the microstructure and mechanical behavior of enamel. We hypothesize that mimicking enamel near the dentin-enamel junction (DEJ) can increase the lifetime by enhancing resistance to crack formation and growth as well as subsequent material loss or bacterial penetration and recurrent decay. In particular, we hypothesize that imitating the enamel columns and the disoriented crossed rods between them can increase toughness while enhancing strength relative to the biting surface. Further, we believe that including stiffer fille materials like titania can enhance load transfer to the remaining tooth which would reduce fracture [and formation of marginal gaps] by decreasing the stress carried by the composite. [In addition, we will seek to minimize marginal gap formation by 'tuning' filler and matrix composition to better match the coefficient of thermal expansion of the tooth.] Our program is novel in its approach to mimicking enamel structure and in the type of composites we propose to develop. Unlike previous attempts to mimic enamel structure that focused on the controlled growth of hydroxyapatite crystals outside the mouth, we propose a system that develops its structure in-situ. Further, unlike commercial composites that have dispersed non-organized filler particles we propose an entirely new class of composites with hierarchically organized filler particles. Our approach will involve synthesizing and functionalizing silica and titania nanorods in low shrinkage phosphate or siloxane based acrylic liquid crystal monomers. These nanorods will be organized into bundles that imitate enamel prisms and then self-assembled into larger ordered structures together with additional discrete filler rods and particles in monomers that wil be subsequently solidified during polymerization. The organization of rods into bundles and then into larger structures will be controlled by thermodynamics and interfacial chemistry through the functionalization process and shape anisotropy. We will [iteratively design the composite microstructure and composition and] validate the potential of the composites as future restoration materials, by systematically assessing the polymerization shrinkage, swelling in a simulated oral environment and mechanical properties including flexural strength, elastic modulus, storage and loss modulus, and toughness. [In addition, we will evaluate the interactions of S mutans, the bacteria typyically responsible for secondary caries to determine if the composites have anti-bacterial properties of if the bacteria degrades the composite.] Because the approach is so new, we are requesting an exploratory grant to develop the enabling techniques required for this new class of highly filled (~60 vol%) composites. Our objective is to demonstrate the feasibility of our approach from a fundamental science, engineering and dental perspective.